Various embodiments of space heaters for motor vehicles and other mobile units have been known. A distinction is made basically between space heaters using air as the heat carrier and space heaters using water as the heat carrier. Another classification is based on the type of the burner. Practically all types of burners have a combustion air fan.
To control the amount of heat supplied by the operation of the space heater, the burner can be switched on and off at certain time intervals. If the heating capacity required is relatively low, it is more favorable for economic reasons and for reasons of environmental protection to operate the burner in partial load operation only. A series resistor can be inserted into the circuit of the electric motor, so that the electric motor and consequently also the fan wheel will only rotate at reduced speed only.
It has hitherto been common practice to arrange this resistor for the partial load operation in the feed lines leading to the electric motor, outside the housing of the unit. The resistor was exposed there to harmful environmental effects (water, dirt), and even to the risk of possible mechanical damage. It was therefore necessary to protect this resistor.
The electric motor has a considerable electric power consumption even in the case of a space heater for relatively low heating capacity. The difference between the full power and the partial power of the electric motor must be converted by the electric resistor into heat during partial load operation. Therefore, a relatively large resistor with large surface was usually used in order to rapidly remove the heat generated.
It has been known especially in connection with liquid fuel heating devices, especially vehicle space heaters, that an electric motor rotating a fan wheel is optionally operated in full load operation and in partial load operation by means of a series resistor. DE 29,31,936 A1 contains the special suggestion that the glow plug intended for the ignition process, which is arranged in the wall of the burner housing, be used as the series resistor. The glow plug arranged downstream of the burner is arranged in the multiwall housing such that part of the glow plug is located in the heating air stream, another part of the glow plug is located in the waste gas stream, and the tip of the glow plug extends into the burner chamber.
It can be determined from the prior-art FR-PS 2,302,426 that an electric resistor serving to control the speed of an electric motor in an electric fan drive is arranged in a common housing with the driving electric motor and in the air stream of a cooling fan. Such an arrangement is expensive and is not applicable in a vehicle auxiliary heater.